The logging of geological formations is, as is well known, economically a highly important activity. The invention is of benefit in logging activities potentially in all kinds of mining and especially in the logging of reserves of oil and gas, water or other valuable commodities.
Virtually all commodities used by mankind are either farmed on the one hand or are mined or otherwise extracted from the ground on the other, with the extraction of materials from the ground providing by far the greater proportion of the goods used by humans.
It is extremely important for an entity wishing to extract materials from beneath the ground to have as good an understanding as possible of the lithology of a region from which extraction is to take place.
This is desirable partly so that an assessment can be made of the quantity and quality, and hence the value, of the materials in question; and also because it is important to know whether the extraction of such materials is likely to be problematic.
In consequence a wide variety of logging methods has been developed over the years. The logging techniques exploit physical and chemical properties of a formation usually through the use of a logging tool or sonde that is lowered into a borehole (that typically is, but need not be, a wellbore) formed in the formation by drilling.
Typically the sonde sends energy into the formation and detects the energy returned to it that has been altered in some way by the formation. The nature of any such alteration is processed into electrical signals that are then used to generate logs (i.e. numerical, graphical or tabular representations containing much data about the formation in question).
The signals may be thought of as log data, or at least as representing log data. These data may be stored, processed and transmitted as numerical values e.g. in a database or other data memory format; or they may be processed to form graphical logs referred to as image logs.
In general the values of the log data when converted to forms that are useable in image logs are represented by features of the image logs that a skilled analyst can interpret by eye in order to assess the conditions in the downhole (i.e. subterranean) environment.
Taking the specific example of resistivity log data, when these are converted to image format the data values are mapped to colours that may then be viewed e.g. on a computer screen or in a colored print-out. It has been found that the use of colours in this way is somewhat intuitive for human analysts to study. In a resistivity image log that has not undergone any processing that alters the scale from place to place in the image therefore the analyst can know with confidence that all regions of the image that are the same color represent regions, of the formation, that are of the same resistivity.
Log data from other types of logging tool may be similarly processed in order to give rise to other kinds of image log.
In short therefore an image log appears as lines and regions of color that represent the shapes and distributions of subterranean features.
It is important in this regard to recognise that the logging sondes do not themselves either illuminate the subterranean formations or indeed capture light-based images. Rather, the log data (that by reason of resulting from penetration by the emitted energy of the geological features surrounding the borehole may relate to locations spaced some distance into the rock from the borehole) may be processed into images in order to facilitate interpretation of the data.
A topic of growing interest is that of machine assessment of log data. In particular the Applicant has pioneered numerous techniques that assist to automate processes of image log interpretation, thereby saving significantly on costs and time. In particular the Applicant has developed highly effective techniques that render image logs suitable for automated analysis; and also has developed highly effective automatic feature recognition and analysis methods.
Despite these advances however some aspects of log data and image logs derived from them remain sub-optimal, as explained below. The methods of the invention seek to improve the processing of image log data so that they more readily can be interpreted (whether by automated machinery such as computers, or by humans).
In part the difficulties derive from techniques of normalization (i.e. the assignment of the log data values to the range of colours available for inclusion in an image log) that are currently in use.
Normalization in the context of image log processing is the transformation of the measured values to a color scale. In respect of micro-resistivity images the convention is to represent low resistivity values by black and high resistivity values by white, with a gradational “earth-tones” color scale between. Typical scales comprise 64 or 128 colours (the human eye having limited ability to differentiate between adjacent colors on gradational scales of more than 128 colours).
Normalization techniques fall broadly into two categories: static normalization and dynamic normalization.
In static normalization a fixed relationship exists between resistivity values and color defined for the entire well. The desired maximum and minimum resistivity values for the scale are defined, and the interval is divided either linearly or logarithmically into the number of bins in the color scale. Static scales allow the gross similarities between intervals to be readily identified. The disadvantage is that the resistivity of rocks spans several orders of magnitude, so small but significant variations in resistivity within a narrow band are not rendered with this method.
As the name implies, in dynamic normalization a dynamic relationship exists between resistivity values and colors in which the color mapping applied to each depth frame is driven by the range of resistivity values in a depth window centered on the depth. The depth window is typically of the order of 1 m. All 64 (or 128) colors are used within each window, so the small but significant variations in resistivity that are lost in static images are rendered in a dynamic image. The disadvantage is that the same color from different parts of the same well will not in general correspond to the same resistivity value.
In more detail, dynamic normalization may be implemented (for example) by computing the mean and standard deviation of the resistivity values in a window, then mapping the color range to the resistivity range (mean−n×standard deviation) to (mean+n×standard deviation). A common alternative dynamic normalization (referred to as equalization) redistributes resistivity values within the depth window so as to achieve a linear cumulative distribution of normalized resistivity values.
A further aspect of dynamic normalization is that none of the prior art dynamic normalization methods takes account of the spatial distribution of the resistivity values within the window. This means that the color mapping applied to the depth frame at the central depth in the window is influenced by resistivity values elsewhere in the window, and this has the potential to create ghosts when the window contains high-contrast transitions. As used herein a “ghost” is a region of an image log created using one of the artificial log data to color mapping techniques in which one color may inaccurately spread into a region of the image that does not exhibit the log data value in question. Ghosting therefore leads to significant inaccuracies, and the loss of some features from the image logs altogether.
It is important to recognise some further features of image logs that while not necessarily disadvantages must be borne in mind when considering them.
Firstly image logs are usually colored, with (as explained) the colors signifying data values such as micro-resistivity values that either apply over the entirety of an image log (in the case of a statically normalized log) or within respective depth windows (in the case of dynamically normalized logs).
Image logs however can with some success be rendered in grayscale. The images herein are so rendered but it is to be understood that the method of the invention is not limited to the production of grayscale images and instead is intended to be applicable to, and indeed is primarily applicable to, colored image logs.
Secondly if it is required to produce logs that are suitable for automated (machine) processing it is desirable to carry out some form of pre-processing on the image logs to render them suitable for this use.
An example of the kind of pre-processing that the Applicant has developed is so-called “in-painting.”
In-painting is a technique that is applicable to resistivity logs, and to logs created from multiple sub-components that need not necessarily be logged at one and the same time.
An example of a logging tool type is the so-called multi-pad micro-resistivity borehole imaging tool, such as the tool 10 illustrated in transversely sectioned view in FIG. 1. In this logging tool a circular array of (in the example illustrated) eight pads 11 each in turn supporting typically two lines of surface-mounted resistivity electrodes referred to as “buttons” 12 is supported on a series of calliper arms 13 emanating from a central cylinder 14. During use of the tool 10 the arms 13 press the buttons 12 into contact with the very approximately cylindrical wall of a borehole. The borehole is normally filled with a fluid (such as a water-based mud) that if conductive provides an electrical conduction path from the formation surrounding the borehole to the buttons.
Many variants on the basic imaging tool design shown are known. In some more or fewer of the pads 11 may be present. The numbers and patterns of the buttons 12 may vary and the support arms also may be of differing designs in order to achieve particular performance effects. Sometimes the designers of the tools aim to create e.g. two parallel rows of buttons located on the pad one above the other. The buttons in the lower row are offset slightly to one side relative to their counterparts in the row above. When as described below the signals generated by the buttons are processed the outputs of the two rows of buttons are in effect lain over one another. As a result the circumferential portion of the borehole over which the buttons 12 of a pad 11 extend is logged as though there exists a single, continuous, elongate electrode extending over the length in question.
In general in operation of a tool such as resistivity tools 10 electrical current generated by electronic components contained within the cylinder 14 spreads into the rock and passes through it before returning to the pads 11. The returning current induces electrical signals in the buttons 12.
Changes in the current after passing through the rock may be used to generate measures of the resistivity or conductivity of the rock. The resistivity data may be processed according to known techniques in order to create (typically colored) image logs that reflect the composition of the solid and fluid parts of the rock. These image logs convey much data to geologists and others having the task of visually inspecting and computationally analyzing them in order to obtain information about the subterranean formations.
In use of a tool such as that shown in FIG. 1 the tool is initially conveyed to a chosen depth in the borehole before logging operations commence. The deployed location may be many thousands or tens of thousands of feet typically but not necessarily below, and in any event separated by the rock of the formation from, a surface location at which the borehole terminates.
Various means for deploying the tools are well known in the mining and oil and gas industries. One characteristic of most if not all of them is that they can cause a logging tool that has been deployed as aforesaid to be drawn from the deployed location deep in the borehole back towards the surface location. During such movement of the tool it logs the formation, usually continuously. As a result the image logs may extend continuously for great distances.
Although the logs are continuous in the longitudinal sense, notwithstanding the pad offsetting explained above they are azimuthally interrupted by reason of the pads not extending all the way continuously around the circumference of the borehole. The design of the tool prevents this since the arms 13 must be extensible in order to press the pads 11 into contact with the borehole wall. Following extension of the arms there exists a series of gaps between the ends of the pads.
No data can be logged in these gaps, which manifest themselves as elongate spaces in the image logs. Example of image logs including several of these gaps or discontinuities are visible in FIGS. 6 and 7a. The discontinuities extend from one end of the reconstructed image log to the other, a distance in some cases of thousands of feet.
Filling in the missing data is advantageous for obvious reasons of the desirability of completeness of information. Moreover it is likely to be required when it is desired to process the image logs using automatic pattern recognition programs in order to try and identify certain features in the logs.
Patent application no. GB 1210533.4 discloses an in-painting technique, the purpose of which is to fill in, in a realistic manner, the discontinuities that result from use of a resistivity imaging tool of the kind illustrated in FIG. 1.
Such in-painting is an example of the kind of pre-processing technique that may be applied to image log data.